1. The Field of the Invention
The present invention relates generally to the manufacture of titanium dioxide nanoparticles and nanoparticle suspensions. In particular, the titanium dioxide nanoparticles and nanoparticle suspensions are made using a dispersing agent to influence the size, shape and/or stability of the nanoparticles.
2. The Relevant Technology
Titanium dioxide is used in a variety of applications including pigmentation, light blocking, UV absorption, anti-microbial, photocatalytic oxidation, optics, semiconductors, polymer fillers, ceramics, and others. In most of these applications, particular crystal structures and particle sizes are needed to provide the desired properties of the titanium nanoparticles. In some cases, particle sizes such as 3-4 nm are ideal for providing proper catalytic activity. In other cases, larger particles such as 10-100 nm or 100-200 nm are more desirable.
Crystal structure can also be important for obtaining desired function. Two commonly used crystal structures of titanium dioxide include anatase and rutile. Anatase is more rare. Titanium dioxide in the anatase crystal structure is often used as a photocatalyst. Examples of reactions that use the anatase crystal include photodecomposition of acetone, phenol or trichloro ethylene; oxidation systems of nitric oxide such as nitrogen mono-oxide and nitrogen dioxide; and conversion systems of solar energy. Titanium dioxide with the rutile crystal structure has been widely used for white pigment materials because of its good light scattering effect, which can protect against ultraviolet light. The rutile crystal has also been used for optical coatings, beam splitters, and anti-reflection coatings, since rutile has a high dielectric constant, high refractive index, good oil adsorption, tinting power, and chemical stability, even under strongly acidic or basic conditions.
Titanium dioxide is also used in humidity sensors and high temperature oxygen sensors. Titanium dioxide is particularly well suited for these industries because of its good chemical stability in the presence of water and its ability to exhibit different electrical characteristics as the partial pressure of oxygen varies.
While the use of titanium dioxide has become pervasive, its manufacture can be difficult and expensive. The difficulties and expenses often occur because of the need to form titanium dioxide particles with a particular size and shape. Because of the high surface energies associated with extremely small particles, there is a tendency for these particles to agglomerate and grow, such that the benefit of small and uniform particle size is lost.
Another problem with the titanium dioxide materials of the prior art is the inability to suitably stabilize suspensions of titanium dioxide nanoparticles. For many uses it would be desirable to have a stable suspension of nanoparticles that could be used days later, or preferably even weeks or months later. Existing titanium dioxide particles and solutions are often too unstable to be economically feasible for use in suspension form. Consequently expensive drying and reconstituting steps are needed to use many existing titanium dioxide products.
Therefore, what is needed are titanium dioxide nanoparticles with controlled particle size, shape, and/or stability and methods for economically producing these particles.